Tunable microwave devices with auto-adjusting matching circuit

ABSTRACT

An impedance matching circuit includes a conductor line having an input port and an output port, a ground conductor, a tunable dielectric material positioned between a first section of the conductor line and the ground conductor, a non-tunable dielectric material positioned between a second section of the conductor line and the ground conductor, and means for applying a DC voltage between the conductor line and the ground conductor. The impedance matching circuit may alternatively include a first planar ground conductor, a second planar ground conductor, a strip conductor having an input port and an output port, and positioned between the first and second planar ground conductors to define first and second gaps, the first gap being positioned between the strip conductor and the first planar ground conductor and the second gap being positioned between the strip conductor and the second planar ground conductor. A non-tunable dielectric material supports the first and second planar ground conductors and the strip conductor in the same plane. A connection is provided for applying a DC voltage between the strip conductor and the first and second planar ground conductors. A plurality of tunable dielectric layer sections are positioned between the strip conductor and the first and second planar ground conductors so as to bridge the gaps between the said first and second planar ground conductors and the strip conductor at a plurality of locations, leaving non-bridged sections in between, defining a plurality of alternating bridged and non-bridged co-planar waveguide sections.

CROSS REFERENCE TO RELATED APPLICATION

This application claims the benefit of U.S. Provisional PatentApplication No. 60/219,500, filed Jul. 20, 2000.

FIELD OF THE INVENTION

The invention relates to the field of tunable microwave devices. Morespecifically, the invention relates to impedance matching circuits thatutilize a bias voltage to alter the permittivity of a tunable dielectricmaterial.

BACKGROUND OF THE INVENTION

Microwave devices typically include a plurality of components that mayhave different characteristic impedances. In order to propagate themicrowave signal through the device with minimal loss, the impedances ofthe various components are matched to the characteristic impedance ofthe input and output signal. By transitioning the impedances so that aninput transmission line is matched, most of the available power from theinput is delivered to the device. Historically, impedance matchingtechniques have treated the matching of components with constantcharacteristic impedances to a constant characteristic impedance of theinput line, e.g. to 50 Ω. Multi-stage matching circuits have beenutilized to obtain minimal reflection loss over a specified frequencyrange of operation of a device. Numerous techniques, such as the use ofradial stubs, quarter wave transformers, and multistage matchingcircuits with specific distributions, such as Binomial or Tchebychef,etc., have been developed in order to achieve maximum power transferfrom the input to the device.

However, the characteristic impedance of the tunable components intunable microwave devices is not a constant value. The characteristicimpedance of the tunable component varies over the operating range ofthe device from a minimum to a maximum impedance value. In tunabledielectric devices, a bias voltage applied to tunable dielectricmaterial provides the ability to alter the dielectric constant. Thechange in the dielectric constant provides a variation in the electricalpath length of a microwave signal. As the electrical properties of thetunable dielectric material are varied, the characteristic impedance isalso affected.

In practice, a single characteristic impedance within the tunablecomponents minimum/maximum impedance range is selected. This singleimpedance value is matched using one of the state of the art impedancematching techniques. However, as the tunable microwave device isoperated, the impedance of the tunable component varies from the matchedimpedance and a degradation in the impedance match occurs.

Prior tunable dielectric microwave transmission lines have utilizedtuning stubs and quarter wave matching transformers to transition theimpedance between the input and output. The technique is best formatching a fixed impedance mismatch. U.S. Pat. No. 5,479,139 by Koscicaet al. discloses quarter wavelength transformers using non-tunabledielectric material for the purpose of impedance matching to aferroelectric phase shifter device. Similar impedance matchingconfigurations using non-tunable dielectric substrate of backgroundinterest are shown in U.S. Pat. No. 5,561,407, U.S. Pat. No. 5,334,958,and U.S. Pat. No. 5,212,463. The disadvantage of the above technique isthat the impedance match is optimal at one selective tuning point of thedevice and degrades as the device is tuned through its range. Hence, thereflection loss due to impedance match increases when the device istuned away from the matched point.

Another impedance matching approach for tunable devices is presented inU.S. Pat. No. 5,307,033 granted to Koscica et al. That patent disclosesthe use of spacing of a half wavelength between elements or matchingnetworks for the purpose of impedance matching.

Still another approach utilizes quarter wavelength transformers ontunable dielectric material as disclosed in U.S. Pat. No. 5,032,805,granted to Elmer et al. Other impedance matching configurations areshown in U.S. Pat. Nos. 6,029,075; 5,679,624; 5,496,795; and 5,451,567.Since it is also desirable to reduce the insertion loss of the matchingnetwork, a disadvantage of the above approach is that the quarterwavelength transformer on tunable dielectric material increases theinsertion loss.

The disclosures of all of the above-mentioned patents are expresslyincorporated by reference.

It would be desirable to minimize the impedance mismatch in tunablemicrowave device applications. There is a need for a technique forimproving impedance matching for tunable microwave components thatachieves minimal reflection and insertion losses throughout the range ofoperation of tunable devices.

SUMMARY OF THE INVENTION

This invention provides an impedance matching circuit comprising aconductive line having an input port and an output port, a groundconductor, a tunable dielectric material positioned between a firstsection of the conductive line and the ground conductor, a non-tunabledielectric material positioned between a second section of the conductorline and the ground conductor, and means for applying a DC voltagebetween the conductive line and the ground conductor.

The invention further encompasses an impedance matching circuitcomprising a first ground conductor, a second ground conductor, a stripconductor having an input port and an output port. The strip conductoris positioned between the first and second ground conductors and todefine first and second gaps, the first gap being positioned between thestrip conductor and the first ground conductor and the second gap beingpositioned between the strip conductor and the second ground conductor.A non-tunable dielectric material supports the first and second groundconductors and the strip conductor in a plane. A connection point isprovided for applying a DC voltage between the strip conductor and thefirst and second ground conductors. A plurality of tunable dielectriclayer sections are positioned between the strip conductor and the firstand second ground conductors so as to bridge the gaps between the firstand second ground conductors and the strip conductor at a plurality oflocations, leaving non-bridged sections in between, defining a pluralityof alternating bridged and non-bridged co-planar waveguide sections.

The matching circuits form tunable impedance transformers that are ableto match a constant microwave source impedance connected at the inputport to a varying load impedance connected at the output port, therebyreducing signal reflections between the microwave source and a variableload impedance.

This invention provides an impedance matching circuit capable ofmatching a range of impedance values to a tunable microwave device inorder to reduce reflections from impedance mismatch during tuning of themicrowave device.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a plan view of a first embodiment of the auto adjustingmatching network of this invention in the form of the microstrip;

FIG. 2 is a cross-sectional view of the embodiment of FIG. 1 taken alongline 2—2, showing a microstrip line geometry;

FIG. 3 is a cross-sectional view of the embodiment of FIG. 1 taken alongline 3—3, showing a microstrip line geometry;

FIG. 4 is a plan view of a second embodiment of the auto adjustingmatching network of this invention in the form of the stripline;

FIG. 5 is a cross-sectional view of the embodiment of FIG. 4 taken alongline 5—5, showing a stripline geometry;

FIG. 6 is a cross-sectional view of the embodiment of FIG. 4 taken alongline 6—6, showing a stripline geometry;

FIG. 7 is a cross-sectional view of a third embodiment for the autoadjusting matching network of this invention based on a coaxialgeometry;

FIG. 8 is a cross-sectional view of the embodiment of FIG. 7 taken alongline 8—8, showing the coaxial transmission line geometry;

FIG. 9 is a plan view of another embodiment for the auto adjustingmatching network of this invention including multiple partial stages ontunable material;

FIG. 10 is a plan view of another embodiment for the auto adjustingmatching network of this invention based on a slotline or finlinegeometry, and including multiple partial stages;

FIG. 11 is a cross-sectional view of the embodiment of FIG. 10 takenalong line 11—11, showing a slotline geometry;

FIG. 12 is a plan view of another embodiment for the auto adjustingmatching network of this invention based on a co-planar waveguidegeometry, and including multiple partial stages;

FIG. 13 is a cross-sectional view of the embodiment of FIG. 12 takenalong line 13—13, showing a coplanar waveguide geometry; and

FIG. 14 is a block diagram showing a matching network of this inventioncoupled to a tunable dielectric device.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The preferred embodiments described herein are each designed for usewithin a certain arbitrary frequency range. For this reason allreferences to a “wavelength” will refer to the center frequency of thedesign.

Referring to the drawings, FIG. 1 is a plan view of a first embodimentof an auto adjusting matching network of this invention in the form ofthe microstrip circuit 10. FIG. 2 is a cross-sectional view of FIG. 1taken along line 2—2, showing the microstrip line geometry. FIG. 3 is across-sectional view of FIG. 1 taken along line 3—3.

The device has two ports 12 and 14 for input and output of a guidedelectromagnetic wave. It includes a multi-stage microstrip line 16,having sections 18 and 20 of various widths and lengths, deposited on anon-tunable dielectric substrate 22, which in turn is supported byground plane 24; and a microstrip line section 26 deposited on a voltagetunable dielectric substrate 28 which in turn is supported by groundplane 30. A biasing electrode 32 in the form of a high impedancemicrostrip line is connected to microstrip section 20.

The biasing electrode 32 serves as a means for connecting an externalvariable DC bias voltage supply 34 to the auto-adjusting impedancematching circuit. The connection of the biasing electrode 32 to thecircuit is not limited to microstrip section 20, but may be made to anyother part of the circuit that is electrically connected to microstripline section 26. Ground planes 24 and 30 are electrically connected toeach other. While ground planes 24 and 30 are shown as separateelements, it should be understood that they may alternatively beconstructed as a single ground plane.

The microstrip line section 26, which comprises a conducting strip, isdirectly supported by a dielectric layer 28, which is the voltagetunable layer. A ground plane 30 supports the dielectric layer 28. Themicrostrip line 26 is less than a quarter wavelength long and forms anapproximately quarter wavelength long transformer when joined to section20 of the matching network on the non-tunable dielectric substrate 22.

The non-tunable stages 36 and 38 of the matching network 10 form amultistage matching circuit 40 directly supported by the non-tunabledielectric layer 22. The multi-stage matching circuit 40 can be anynumber of stages of varying widths and lengths, not limited to quarterwavelength sections. If the non-tunable and tunable substrates 22 and 28respectively are not of the same height, the last stage 38 of thematching network, which abuts the tunable dielectric 28, would beelectrically connected to microstrip line section 26 via a step 42.

FIG. 4 is a plan view of a second embodiment of the auto adjustingmatching network 44 of the invention in the form of a stripline. FIG. 5is a cross-sectional view of FIG. 4 taken along line 5—5, showing astripline geometry. FIG. 6 is a cross-sectional view of FIG. 4 takenalong line 6—6.

The device 44 has two ports 46 and 48 for input and output of the guidedelectromagnetic wave. It comprises a stripline 50 having sections 52 and54 of various widths and lengths embedded in a non-tunable dielectricsubstrate 56 supported by top and bottom ground planes 58 and 60, anadditional section 62 of stripline 50 embedded in a tunable dielectricsubstrate 64 supported by top and bottom ground planes 66 and 68, and abiasing electrode 70 in the form of a high impedance stripline isconnected to stripline section 54.

The connection of biasing electrode 70 to the circuit is not limited tostripline section 54, but may be made to any other part of the circuitthat is electrically connected to stripline section 62. The biasingelectrode 70 serves as means for connecting the auto adjusting impedancematching circuit to an external adjustable DC voltage bias source 72.Ground planes 66 and 68 may or may not be the same ground planes as forthe tunable microwave device to which the matching circuit is connected.Ground planes 66 and 68 are electrically connected to the ground planesof the tunable microwave device. Ground planes 66 and 68 areelectrically connected to ground planes 58 and 60.

The stripline section 62, which is a conducting strip, is directlyembedded in the tunable dielectric layer 64, which is the voltagetunable layer. Ground planes 66 on the top and 68 on the bottom supportthe dielectric layer 64. The stripline 62 is less than a quarterwavelength long and forms an approximate quarter wavelength longtransformer when joined to section 54 of the matching network in thenon-tunable dielectric substrate 56.

The non-tunable stages 76 and 78 of the matching network 44 form amultistage matching circuit 80 directly supported by the non-tunabledielectric layer 56. The multi-stage matching circuit 80 can be anynumber of stages of varying widths and lengths, not limited to andincluding quarter wavelength sections. The last stage 78 of the matchingnetwork, which abuts the tunable dielectric 64, is electricallyconnected to microstrip line section 62.

FIG. 7 is a longitudinal cross-sectional view of a third embodiment ofthe invention for the auto adjusting matching network based on a coaxialgeometry. FIG. 8 is a cross-sectional view of FIG. 7 taken along line8—8, showing the coaxial transmission line geometry.

The device 82 of FIGS. 7 and 8 has two ports 84 and 86 for input andoutput of the guided electromagnetic wave. It comprises a centerconductor 88 having sections 90 and 92 of various diameters and lengthssurrounded by a non-tunable dielectric substrate 94, which in turn issurrounded by ground conductor 96. An additional center conductorsection 98 is surrounded by a tunable dielectric substrate 100, which inturn is surrounded by ground conductor 102. A thin biasing electrode 104enters the co-axial structure through a small hole 106 and is connectedto the central conductor 88.

The connection of biasing electrode 104 to the circuit is not limited tothe center conductor section 92, but may be made to any other part ofthe circuit that is electrically connected to center conductor section98. The biasing electrode 104 serves as a means for connecting anexternal adjustable DC voltage bias source 74 to the auto-adjustingimpedance matching circuit. Ground conductor 102 may or may not be thesame ground conductor as for a tunable microwave device to which thematching circuit would be connected. Ground conductor 102 iselectrically connected to the ground conductor of the tunable microwavedevice. Ground conductors 96 and 102 are electrically connected to eachother.

The center conductor section 98 is surrounded by a dielectric layer 100,which is the voltage tunable layer. The voltage tunable dielectric layer100 is enclosed by a ground conductor 102. The center conductor section98 is less than a quarter wavelength long and forms a compositeimpedance transformer approximately a quarter wavelength long whenjoined to section 92 of the matching network in the non-tunabledielectric 94.

The matching network 109 is a multi-stage matching circuit surrounded bya dielectric layer 94, which is a non-tunable dielectric. The dielectriclayer 94 is enclosed by a ground conductor 96. The multi-stage matchingcircuit can be any number of stages of varying widths and lengths, notlimited to and including quarter wavelength sections. The last stage 108of the matching circuit, which abuts the tunable dielectric 100, iselectrically connected to the center conductor 98.

An extension to the first embodiment is shown in FIG. 9, as a matchingcircuit having multiple tunable stages 112, 114, 116, and multiplenon-tunable stages 154, 156, 114 a and 116 a. The device has two ports118 and 120 for input and output of a guided electromagnetic wave. Itincludes a matching microstrip line section 122, deposited on anon-tunable dielectric substrate 124; multiple pairs of microstripsections 126, 128 and 130, 132 and 134, 136 deposited on pairs ofnon-tunable and tunable dielectric substrates 124, 138 and 140, 142 and144, 146 respectively; a biasing electrode 148 in the form of a highimpedance microstrip line connected to microstrip section 126; and aground plane (not shown). The dielectric substrates are supported by theground plane, which may include different electrically connectedsections to adapt to the different thicknesses of the substrates 124,140 144 and 138, 142, 146.

The connection of biasing electrode 148 to the circuit is not limited tomicrostrip section 126, but may be made to any other part of the circuitthat is electrically connected to microstrip line sections 128, 132 and136. The biasing electrode 148 connects the auto-adjusting impedancematching circuit to an adjustable DC voltage bias source 152. The groundplane may or may not be the same ground plane as for the tunablemicrowave device to which the matching circuit is connected.

Each of the microstrip line sections 128, 132 and 136 is less than aquarter wavelength long and forms an approximately quarter wavelengthlong impedance transformer when joined to microstrip sections 126, 130and 134 respectively.

The non-tunable stages of the matching network 154, 156 form amulti-stage matching circuit 158 directly supported by the non-tunabledielectric layer 124. The multistage matching circuit can be any numberof stages of varying widths and lengths, not limited to quarterwavelength sections. Microstrip section 126 of the last stage 156 of thenon-tuning part of the matching network, which abuts the tunable stage112, is electrically connected to microstrip line section 128. Thelatter abuts non-tunable tunable stage 114 a and is electricallyconnected to microstrip line section 130. The latter abuts tunable stage114 and is electrically connected to microstrip line section 132. Thelatter abuts non-tunable tunable stage 134 and is electrically connectedto microstrip line section 134. The latter abuts tunable stage 116 andis electrically connected to microstrip line section 136.

The multiple stage pairs in FIG. 9 ensure impedance matching over awider frequency and impedance range than the more simple geometry ofFIG. 1. It should be understood that a similar extension to multiplestage pairs can be made for the second (stripline) and third (co-axial)embodiments as well.

FIG. 10 is a plan view of another embodiment for the auto adjustingmatching network 160 based on a slotline or finline geometry, andincluding multiple partial stages 162, 164, and 166. FIG. 11 is across-sectional view of FIG. 10 taken along line 11—11, showing aslotline geometry.

The device 160 has two ports 168 and 170 for input and output of theguided electromagnetic wave. It includes two conducting coplanarconductors 172 and 174, supported by non-tunable dielectric layer 176,and separated by a gap 178 to form a slotline (or finline if integratedinto a waveguide) geometry. For comparison with the first, second andthird embodiments, one of these coplanar conductors 174 can beconsidered to be the ground conductor. The slot 178 may be of uniformwidth, or it can be of non-uniform width as shown in FIG. 10. Atmultiple locations (three shown in FIG. 10) 162, 164 and 166, the slotis bridged by a tunable dielectric layer 180, 182 and 184, which can bedeposited on the supporting dielectric layer 176 using thick or thinfilm technology prior to depositing the metal layers 172 and 174 on thesupporting dielectric layer 176. Between locations 162, 164 and 166,there remain sections 186 and 188 as well as 190 a and 190 b, which arenot bridged with a tunable material layer. Planar conductor 172 isconnected to an adjustable DC voltage bias source 192, and planarconductor 174 is connected to DC ground. The coplanar conductors may ormay not be the same coplanar conductors as for the tunable microwavedevice to which the matching circuit is connected.

Each of the tunable slotline sections 162, 164 and 166 is less than aquarter wavelength long, but together with the non-tunable intermediatesections 186, 188, 190 a and 190 b which are also typically shorter thana quarter wavelength long, these cascaded slotline sections form acascaded network that may be a multiple of quarter wavelengths long. Thenetwork can be made longer by simply adding more pairs of tunable andnon-tunable slotline sections. By careful choice of the relative lengthsof each tunable and non-tunable slotline section, the cascaded networkforms a tunable impedance matching network over a wide frequency band.

The slotline sections 186, 188 190 a and 190 b may also be bridged by atunable layer, similar to the tunable sections 162, 164 and 166, butwhich may be less tunable. Reduced tunability in regions 186, 188 190 aand 190 b can be achieved by using a material that is less tunableand/or by using wider slot gaps to reduce the bias field strength inthese regions. Instead of using different types of materials in thestrongly tunable and lesser tunable slot regions, a tunable material canbe deposited which may have varying tunability along the slot length.

FIG. 12 is a plan view of another embodiment for the auto adjustingmatching network 194 based on a co-planar waveguide geometry, andincluding multiple partial stages. FIG. 13 is a cross-sectional viewtaken along line 13—13 in FIG. 12, showing a coplanar waveguidegeometry.

The device 194 has two ports 196 and 198 for input and output of theguided electromagnetic wave. It includes two coplanar conducting groundconductors 200, 202 and a central strip conductor 204, supported bynon-tunable dielectric layer 206, and separated by gaps 208 and 210 toform a co-planar waveguide geometry. The slots 208 and 210 may be ofuniform width, or they can be of non-uniform width as shown in FIG. 12.At multiple locations 212, 214 and 216, the slots 208 and 210 arebridged by a tunable dielectric layers 218 220 and 222, which can bedeposited on the supporting dielectric layer 206 using thick or thinfilm technology prior to depositing the metal layers 200 and 202 and 204on the supporting dielectric layer 206. Between locations 212, 214 and216, there remain sections 224, 226 as well as 228 a and 228 b, whichare not bridged with a tunable material layer. Strip conductor 204 isconnected to an adjustable DC voltage bias source 230, and planar groundconductors 200 and 202 are connected to DC ground. The coplanarconductors 200 and 202 and strip 204 may or may not be the same coplanarconductors as for the tunable microwave device to which the matchingcircuit is connected.

Each of the tunable co-planar waveguide sections 212, 214 and 216 isless than a quarter wavelength long, but together with the non-tunableintermediate sections 224, 226, 228 a and 228 b, which are alsotypically shorter than a quarter wavelength long, these cascadedco-planar waveguide sections form a cascaded network that may be amultiple of quarter wavelengths long. The network can be made longer bysimply adding more pairs of tunable and non-tunable co-planar waveguidesections. By careful choice of the relative lengths of each tunable andnon-tunable section, the cascaded network forms a tunable impedancematching network over a wide frequency band.

The slots in co-planar waveguide sections 224, 226, 228 a and 228 b mayalso be bridged by a tunable layer, similar to the tunable sections 212,214 and 216, but in that case the layer may be less tunable. Reducedtunability in regions 224, 226, 228 a and 228 b can be achieved by usinga material that is less tunable and/or by using wider slot gaps toreduce the bias field strength in these regions. Instead of usingdifferent types of materials in the strongly tunable and lesser tunableslot regions, a tunable material can be deposited which may have varyingtunability along the co-planar waveguide length.

FIG. 14 is a block diagram showing a matching network 10 constructed inaccordance with this invention coupled to a tunable microwave device232. The tunable microwave device 232 could be one of many devices whichhave varying input/output characteristic impedances such as tunablephase shifters, delay lines, filters, etc. In the arrangement shown inFIG. 14, the adjustable external DC voltage source is used to supplybias voltage to the matching network 10 and the tunable microwave device232 in tandem. As the voltage supplied by the external DC voltage sourcechanges, the characteristic input/output impedance of the tunabledielectric device will also change. At the same time the impedancecharacteristics of the matching network will change to maximize powertransfer from/to the microwave source/load 234 to/from the tunablemicrowave device 232. Alternatively, the tunable microwave device 232and the matching network 10 can be controlled by two different externalDC voltage sources.

The first preferred embodiment of the auto adjusting matching networkuses a microstrip geometry. The second preferred embodiment of theauto-adjusting matching circuit has a stripline geometry, the third hasa coaxial geometry, the fourth has a slotline or finline geometry andthe fifth has a co-planar waveguide geometry.

In some embodiments, this invention provides a multi-stage impedancecircuit functionally interposed between a conductor line and an entrypoint of a tunable microwave device, wherein the multi-stage impedancematching circuit reduces the signal reflection of a microwave signalpropagating through the tunable impedance transformer into the microwavedevice, by matching the wave impedance of a microwave signal at theentry point, to the microwave source impedance.

This invention provides electrically controlled auto-adjusting matchingnetworks that contribute to the tunable applications of microwavedevices, while improving upon the range of operation of such devices. Itovercomes the problem of matching to a microwave transmission line witha varying characteristic impedance. It is well suitable for tunablephase shifters, delay lines, and impedance matching for power amplifiersused as general-purpose microwave components in a variety ofapplications such as handset power amplifiers, radar, microwaveinstrumentation and measurement systems and radio frequency phased arrayantennas. The devices are applicable over a wide frequency range, from500 MHz to 40 GHz.

The invention provides an impedance matching circuit having minimalreflection loss and reduced insertion loss over the tuning range of thedevice.

The auto-adjusting matching circuits of this invention may have a dualfunction. The main objective of the auto-adjusting matching circuit isto operate as an impedance matching network. Additionally, theauto-adjusting matching circuit has the ability to contribute to thetunable range of the microwave device to which it is coupled. Hence, theauto-adjusting matching circuit may incorporate tunable applications inits design as well. For example, the length of a tunable phase shiftermay be decreased since the matching network provides a small amount oftunable phase shift through its operating range. Thus, both objectivesalso lead to a decrease in the insertion loss.

The present invention is advantageous because it has wide application totunable microwave transmission line applications that make use of astatic electric field to produce the desired tuning effect. Thisinvention is also applicable to tunable microwave device applicationsthat operate over a frequency band or at a single frequency.

The auto-adjusting matching circuit according to the present inventionmay or may not contribute to the design criteria of the tunableapplication and may use a common DC Voltage bias or a different DCVoltage bias. The invention minimizes reflection loss and increases theuseable bandwidth of the microwave application.

The auto-adjusting matching circuit is a multi-stage impedance matchingcircuit that includes both non-tunable and tunable dielectric material.For example in one preferred embodiment, the impedance matchingtransformer stage supported by the tunable dielectric material is lessthan a quarter wavelength long and is connected to the adjacenttransformer supported by the non-tunable dielectric to form a compositequarter wavelength impedance transformer. Individual sections of such acomposite quarter wave transformer can be referred to as “partialstages”. The matching transformers are tuned in tandem with themicrowave device, in order to obtain low insertion loss as well asreducing the reflections from impedance mismatch. Thus, minimalinsertion loss in the matching network is achieved. Additionally, thereflections from impedance mismatch due to the tuning of the microwavedevice are also minimized.

The auto-adjusting matching circuit is a two-port device, which in itssimplest form includes a conducting matching network supported by alow-loss, conventional non-tunable dielectric substrate which in turn isa supported by a first ground conductor; at least one conductingpartial-stage supported by a low-loss voltage-tunable dielectric layer,which in turn is supported by a second ground conductor; and a biasingelectrode for connection to an external variable DC voltage source,preferably by way of a microwave choke.

The partial-stage on the tunable dielectric layer and the adjoiningpartial stage on the non-tunable dielectric layer together can form anapproximate quarter wavelength long impedance transformer. The portleading to the partial stage supported by the tunable dielectric can beconnected at the input or output of a tunable microwave device. Theother port of the auto-adjusting matching circuit, which is connected tothe matching section supported by the non-tunable dielectric substrate,forms a microwave signal input/output port, which has a substantiallyconstant characteristic impedance.

The low-loss voltage-tunable dielectric layer of the partial-stage ofthe matching circuit may be comprised of the same tunable dielectricmaterial as the tunable microwave device to which it is connected, or itmay be comprised of a different tunable dielectric material. Theauto-adjusting matching circuit may be biased with the same bias voltageas the tunable microwave device to which it is connected, or it may havea separate bias voltage applied. If more than one tunable material isused, i.e. one tunable dielectric material for the microwave tunabledevice and another for the partial-stage of the matching circuit, eachmay have its own separate bias voltage source or use a common (shared)bias voltage source.

As is well known, the bandwidth of the impedance matching network may beimproved by additional matching stages. The additional matching stagesmay each be comprised of a partial stage supported by a tunabledielectric substrate in series with a partial stage supported by anon-tunable dielectric substrate. The tunable dielectric substratesections for the additional matching stages may be comprised of the sametunable dielectric material as the first tunable partial-stage, or itmay be comprised of a different tunable dielectric material.

The microwave matching section, which is supported by the tunabledielectric substrate for the dual purpose of reducing signal reflectionsand providing good transmission to and from the microwave transmissionline application as well as contributing to the tunability of theapplication, may be a partial stage less than a quarter wave lengthlong, or may include more than one matching section and is not limitedto the use of one tunable dielectric substrate.

The biasing electrode may be connected to the auto-adjusting matchingcircuit by way of a microwave choke such as a high impedancetransmission line, or by a highly inductive wire attached directly tothe auto-adjusting matching circuit at any point that is ultimatelyelectrically connected to the partial stage supported by the tunabledielectric.

The first and second ground conductors are electrically connected and ifboth are of a planar construction, they should preferably form onecontinuous ground plane. The non-tunable matching stages areelectrically connected to the tunable partial-stage.

The objective of a matching network is to ensure that a guidedelectromagnetic wave entering one port (as such defined as the inputport) will enter the microwave device and leave it at the other port(output), with minimum residual reflections at each port. The groundplane is kept at zero voltage, while a voltage bias is applied to theelectrodes. The voltage bias causes a DC electric field across thevoltage tunable dielectric, which affects the dielectric permittivity ofthe medium. Since the characteristic impedance of the microstrip isinversely proportional to the square root of the effective dielectricpermittivity of the medium around the strip, the biasing voltage can beused to control the characteristic impedance of the auto-adjustingmatching network. In this way, the characteristic impedance of theinvention can be controlled by the voltage bias. The advantages of thisinvention are low insertion loss and improved bandwidth operation fortunable devices.

Tunable dielectric materials have been described in several patents.Barium strontium titanate (BaTiO₃—SrTiO₃), also referred to as BSTO, isused for its high dielectric constant (200-6,000) and large change indielectric constant with applied voltage (25-75 percent with a field of2 Volts/micron). Tunable dielectric materials including barium strontiumtitanate are disclosed in U.S. Pat. No. 5,427,988 by Sengupta, et al.entitled “Ceramic Ferroelectric Composite Material-BSTO—MgO”; U.S. Pat.No. 5,635,434 by Sengupta, et al. entitled “Ceramic FerroelectricComposite Material-BSTO-Magnesium Based Compound”; U.S. Pat. No.5,830,591 by Sengupta, et al. entitled “Multilayered FerroelectricComposite Waveguides”; U.S. Pat. No. 5,846,893 by Sengupta, et al.entitled “Thin Film Ferroelectric Composites and Method of Making”; U.S.Pat. No. 5,766,697 by Sengupta, et al. entitled “Method of Making ThinFilm Composites”; U.S. Pat. No. 5,693,429 by Sengupta, et al. entitled“Electronically Graded Multilayer Ferroelectric Composites”; U.S. Pat.No. 5,635,433 by Sengupta entitled “Ceramic Ferroelectric CompositeMaterial BSTO—ZnO”; U.S. Pat. No. 6,074,971 by Chiu et al. entitled“Ceramic Ferroelectric Composite Materials with Enhanced ElectronicProperties BSTO—Mg Based Compound-Rare Earth Oxide”. These patents areincorporated herein by reference.

The electronically tunable materials that can be used in the presentinvention include at least one electronically tunable dielectric phase,such as barium strontium titanate, in combination with at least twoadditional metal oxide phases. Barium strontium titanate of the formulaBa_(x)Sr_(1-x)TiO₃ is a preferred electronically tunable dielectricmaterial due to its favorable tuning characteristics, low Curietemperatures and low microwave loss properties. In the formulaBa_(x)Sr_(1-x)TiO₃, x can be any value from 0 to 1, preferably fromabout 0.15 to about 0.6. More preferably, x is from 0.3 to 0.6.

Other electronically tunable dielectric materials may be used partiallyor entirely in place of barium strontium titanate. An example isBa_(x)Ca_(1-x)TiO₃, where x is in a range from about 0.2 to about 0.8,preferably from about 0.4 to about 0.6. Additional electronicallytunable ferroelectrics include Pb_(x)Zr_(1-x)TiO₃ (PZT) where x rangesfrom about 0.05 to about 0.4, lead lanthanum zirconium titanate (PLZT),PbTiO₃, BaCaZrTiO₃, NaNO₃, KNbO₃, LiNbO₃, LiTaO₃, PbNb₂O₆, PbTa₂O₆,KSr(NbO₃) and NaBa₂(NbO₃)5KH₂PO₄.

In addition, the following U.S. Patent Applications, assigned to theassignee of this application, disclose additional examples of tunabledielectric materials: U.S. application Ser. No. 09/594,837 filed Jun.15, 2000, entitled “Electronically Tunable Ceramic Materials IncludingTunable Dielectric and Metal Silicate Phases”; U.S. application Ser. No.09/768,690 filed Jan. 24, 2001, entitled “Electronically Tunable,Low-Loss Ceramic Materials Including a Tunable Dielectric Phase andMultiple Metal Oxide Phases”; U.S. application Ser. No. 09/882,605 filedJun. 15, 2001, entitled “Electronically Tunable Dielectric CompositeThick Films And Methods Of Making Same”; and U.S. ProvisionalApplication Ser. No. 60/295,046 filed Jun. 1, 2001 entitled “TunableDielectric Compositions Including Low Loss Glass Frits”. These patentapplications are incorporated herein by reference.

The tunable dielectric materials can also be combined with one or morenon-tunable dielectric materials. The non-tunable phase(s) may includeMgO, MgAl₂O₄, MgTiO₃, Mg₂SiO₄, CaSiO₃, MgSrZrTiO₆, CaTiO₃, Al₂O₃, SiO₂and/or other metal silicates such as BaSiO₃ and SrSiO₃. The non-tunabledielectric phases may be any combination of the above, e.g., MgOcombined with MgTiO₃, MgO combined with MgSrZrTiO₆, MgO combined withMg₂SiO₄, MgO combined with Mg₂SiO₄, Mg₂SiO₄ combined with CaTiO₃ and thelike.

Additional minor additives in amounts of from about 0.1 to about 5weight percent can be added to the composites to additionally improvethe electronic properties of the films. These minor additives includeoxides such as zirconnates, tannates, rare earths, niobates andtantalates. For example, the minor additives may include CaZrO₃, BaZrO₃,SrZrO₃, BaSnO₃, CaSnO₃, MgSnO₃, Bi₂O₃/2SnO₂, Nd₂O₃, Pr₇O₁₁, Yb₂O₃,Ho₂O₃, La₂O₃, MgNb₂O₆, SrNb₂O₆, BaNb₂O₆, MgTa₂O₆, BaTa₂O₆ and Ta₂O₃.

Thick films of tunable dielectric composites can compriseBa_(1-x)Sr_(x)TiO₃, where x is from 0.3 to 0.7 in combination with atleast one non-tunable dielectric phase selected from MgO, MgTiO₃,MgZrO₃, MgSrZrTiO₆, Mg₂SiO₄, CaSiO₃, MgAl₂O₄, CaTiO₃, Al₂O₃, SiO₂,BaSiO₃ and SrSiO₃. These compositions can be BSTO and one of thesecomponents or two or more of these components in quantities from 0.25weight percent to 80 weight percent with BSTO weight ratios of 99.75weight percent to 20 weight percent.

The electronically tunable materials can also include at least one metalsilicate phase. The metal silicates may include metals from Group 2A ofthe Periodic Table, i.e., Be, Mg, Ca, Sr, Ba and Ra, preferably Mg, Ca,Sr and Ba. Preferred metal silicates include Mg₂SiO₄, CaSiO₃, BaSiO₃ andSrSiO₃. In addition to Group 2A metals, the present metal silicates mayinclude metals from Group 1A, i.e., Li, Na, K, Rb, Cs and Fr, preferablyLi, Na and K. For example, such metal silicates may include sodiumsilicates such as Na₂SiO₃ and NaSiO₃-5H₂O, and lithium-containingsilicates such as LiAlSiO₄, Li₂SiO₃ and Li₄SiO₄. Metals from Groups 3A,4A and some transition metals of the Periodic Table may also be suitableconstituents of the metal silicate phase. Additional metal silicates mayinclude Al₂Si₂O₇, ZrSiO₄, KalSi₃O₈, NaAlSi₃O₈, CaAl₂Si₂O₈, CaMgSi₂O ₆,BaTiSi₃O₉ and Zn₂SiO₄. Tunable dielectric materials identified asParascan™ materials, are available from Paratek Microwave, Inc. Theabove tunable materials can be tuned at room temperature by controllingan electric field that is applied across the materials.

In addition to the electronically tunable dielectric phase, theelectronically tunable materials can include at least two additionalmetal oxide phases. The additional metal oxides may include metals fromGroup 2A of the Periodic Table, i.e., Mg, Ca, Sr, Ba, Be and Ra,preferably Mg, Ca, Sr and Ba. The additional metal oxides may alsoinclude metals from Group 1A, i.e., Li, Na, K, Rb, Cs and Fr, preferablyLi, Na and K. Metals from other Groups of the Periodic Table may also besuitable constituents of the metal oxide phases. For example, refractorymetals such as Ti, V, Cr, Mn, Zr, Nb, Mo, Hf, Ta and W may be used.Furthermore, metals such as Al, Si, Sn, Pb and Bi may be used. Inaddition, the metal oxide phases may comprise rare earth metals such asSc, Y, La, Ce, Pr, Nd and the like.

The additional metal oxides may include, for example, zirconnates,silicates, titanates, aluminates, stannates, niobates, tantalates andrare earth oxides. Preferred additional metal oxides include Mg₂SiO₄,MgO, CaTiO₃, MgZrSrTiO₆, MgTiO₃, MgAl₂O₄, WO₃, SnTiO₄, ZrTiO₄, CaSiO₃,CaSnO₃, CaWO₄, CaZrO₃, MgTa₂O₆, MgZrO₃, MnO₂, PbO, Bi₂O₃ and La₂O₃.Particularly preferred additional metal oxides include Mg₂SiO₄, MgO,CaTiO₃, MgZrSrTiO₆, MgTiO₃, MgAl₂O₄, MgTa₂O₆ and MgZrO₃.

The additional metal oxide phases may alternatively include at least twoMg-containing compounds. In addition to the multiple Mg-containingcompounds, the material may optionally include Mg-free compounds, forexample, oxides of metals selected from Si, Ca, Zr, Ti, Al and/or rareearths. In another embodiment, the additional metal oxide phases mayinclude a single Mg-containing compound and at least one Mg-freecompound, for example, oxides of metals selected from Si, Ca, Zr, Ti, Aland/or rare earths.

This invention provides minimal loss auto-adjusting matching circuitsfor application to microwave transmission line devices that utilize abias voltage for tuning. Each embodiment of the auto-adjusting matchingcircuit is comprised of a microwave transmission line configuration, atunable dielectric material, means for connecting to a bias voltage, anda non-tunable low-loss dielectric material. In operation, theauto-adjusting matching circuit is placed adjacent to the tunablemicrowave device in order to reduce the reflections from impedancemismatch.

The invention contemplates various dielectric materials, tunabledielectric materials, tunable liquid crystals, bias line geometries,matching stages, impedances of microstrip lines, and operatingfrequencies of the auto-adjusting matching circuit. It should beunderstood that the foregoing disclosure relates to only typicalembodiments of the invention and that numerous modifications oralternatives may be made therein by those skilled in the art withoutdeparting from the scope of the invention as set forth in the appendedclaims.

What is claimed is:
 1. An impedance matching circuit comprising: a firstplanar conductor; a second planar conductor; means for applying a DCvoltage from a DC voltage supply circuit between said first planarconductor and said second planar conductor; a non-tunable dielectricmaterial supporting the first and second planar conductors in the sameplane; a gap between said first and second planar conductors forming aslotline transmission line; a plurality of tunable dielectric layersections, each possessing a dielectric constant that can be varied byapplying a DC electric field, the tunable dielectric layer sectionsbeing interposed between the said co-planar conductors and thenon-tunable dielectric material so as to bridge the gap between thefirst and second planar conductors at a plurality of locations, leavingnon-bridged sections in between, wherein the plurality of alternatingbridged and non-bridged slotline sections form a tunable impedancetransformer circuit with an entry point and an exit point; the saidtunable impedance transformer circuit matching a constant microwavesource impedance connected at the said entry point to a varying loadimpedance connected at the said exit point, thereby reducing signalreflections between the microwave source and the varying load impedance.2. The impedance matching circuit of claim 1, wherein: the first andsecond planar conductors are connected to opposite walls of a waveguideto form a finline tunable impedance transformer circuit.
 3. Theimpedance matching circuits of claim 1, wherein: the non-bridgedsections are bridged by a second tunable dielectric layer.
 4. Theimpedance matching circuits of claim 1, wherein: the bridged sectionshave narrowed slot gaps compared to the said non-bridged sections. 5.The impedance matching circuits of claim 1, wherein: the tunabledielectric layer sections comprise a tunable dielectric layer ofgraduated tunability.
 6. The impedance matching circuit of claim 1,wherein the tunable dielectric sections include a tunable phase and anon-tunable phase.
 7. The impedance matching circuit of claim 6, whereinthe non-tunable phase comprises a metal oxide.
 8. The impedance matchingcircuit of claim 6, wherein the non-tunable phase comprises a metalsilicate.
 9. The impedance matching circuit of claim 1, wherein thetunable dielectric sections include a material selected from the groupof: Ba_(x)Sr_(1-x)TiO₃, Ba_(x)Ca_(1-x)TiO₃, Pb_(x)Zr_(1-x)TiO_(3,) leadlanthanum zirconium titanate, PbTiO₃, BaCaZrTiO₃, NaNO₃, KNbO₃, LiNbO₃,LiTaO₃, PbNb₂O₆, PbTa₂O₆, KSr(NbO₃) and NaBa₂(NbO₃)5KH₂PO₄.
 10. Theimpedance matching circuit of claim 9, wherein the tunable dielectricsections further include a material selected from the group of: MgO,MgAl₂O₄, MgTiO₃, Mg₂SiO₄, MgZrO₃, CaSiO₃, MgSrZrTiO₆, CaTiO₃, Al₂O₃,SiO₂, BaSiO₃ and SrSiO₃, and combinations thereof.
 11. The impedancematching circuit of claim 9, wherein the tunable dielectric sectionsfurther include a material selected from the group of: zirconnates,tannates, rare earths, niobates, tantalates, CaZrO₃, BaZrO₃, SrZrO₃,BaSnO₃, CaSnO₃, MgSnO₃, Bi₂O₃/2SnO₂, Nd₂O₃, Pr₇O₁₁, Yb₂O₃, Ho₂O₃, La₂O₃,MgNb₂O₆, SrNb₂O₆, BaNb₂O₆, MgTa₂O₆, BaTa₂O₆ and Ta₂O₃.